The present invention relates to apparatus for measuring weak magnetic fields with several gradiometers which are arranged, with their associated SQUIDs, in a Dewar vessel which is provided with a relatively narrow neck.
It is known to use superconducting quantum interference devices which are known by the designation "SQUID" (Superconducting Quantum Interference Device) for measuring weak variable magnetic fields, for instance, in a field intensity range below 10.sup.-10 T and in particular, below 10.sup.-12 T. These elements are preferably used in medical technology, especially in magnetocardiography, where magnetic fields of the heart in the order of about 50 pT are measured as well as in magnetoencephalography, where magnetic fields of the brain in the order of about 0.1 pT must be measured. The apparatus essentially comprises a detector coil which, together with a coupling coil, forms a flux transformer. Associated with the SQUID is electronic circuitry for determining and processing signals. Since the magnetic fields to be measured are up to six orders of magnitude smaller than the external interference fields, appropriate shielding is necessary. Because of its greater sensitivity, a DC-SQUID (direct current SQUID) can be used which contains two Josephson junctions. With an associated compension coil, the detector coil forms a so-called gradiometer. With gradiometers of the zero.sup.th, first or higher order, the biomagnetic near field which is still non-uniform in the gradiometer range can be determined selectively.
In order to obtain a three-dimensional field distribution, measurements must be made at successive points in time of the region to be examined. However, the difficulty then arises that the coherence of the field data is no longer assured over the required measuring time and in addition, clinically intolerable measuring times result. While with one known device which contains several parallel measuring channels each with an RF SQUID (Radio Frequency SQUID), the measuring time can be shortened in principle (Physica 107B (1981), pages 29 and 30, North Holland Publishing Co.), a gradiometer array, with which at least one SQUID array is associated, cannot be made with this embodiment.
The SQUIDs and their superconducting components are kept during operation at a temperature of 4.2 K and are immersed for this purpose in liquid helium. The space between the double walls of the Dewar vessel is evacuated and contains exhaust-gas-cooled superinsulation. The heat supply from the outside through the walls of the vessel is therefore small. In order to achieve a long service life of, for instance, several weeks, however, the heat supply through the neck of the Dewar vessel must be limited. Therefore, these Dewar vessels are provided with a relatively narrow neck with integrated exhaust-cooling (Rev. Sci. Instrum. 53 (12) December 1982, pages 1815 to 1845).
The field distribution over a larger area, for instance, in magnetocardiograpy can be determined with a gradiometer array which contains a multiplicity of gradiometers, for instance, 50 or more, which, for instance, form a matrix, and the detector coils of which are arranged side-by side in a plane. Such a gradiometer, however, can no longer be inserted through the narrow neck in the Dewar vessel.